An advance messaging license is required to moderate a space and to add Space announcements.
However we do not have any documentation saying so.

The following documentations only says talks about roles of Moderator but doesn't talk about its requirements:

https://help.webex.com/en-us/article/zuwad5/Webex-App-%7C-Roles-on-Teams-and-Spaces

https://help.webex.com/en-us/article/xuwad5/Webex-App-%7C-Make-someone-a-team-moderator